Bodies constantly sense and humans frequently notice their bodies in order to promote or protect health. Lying at the heart of human life, reproduction has long been entwined with both kinds of sensing. Contemporary fertility biosensing blends techniques of body awareness and related forms of recording (such as diaries and charts) with modern forms of scientific and biomedical knowledge, high-tech gadgets and biodigital platforms. Recent studies show that people who menstruate and/or have ovaries and a uterus use a wide variety of technologies – from paper-based diaries, through mobile phone apps, to wearable biosensing devices – to track and record fertility or sex hormone-related events such as menstruation, ovulation, unprotected heterosexual intercourse and other acts of insemination (Lupton, 2015; Wilkinson et al, 2015; Epstein et al, 2017).

Fertility biosensors articulate bodies that menstruate and/or ovulate through codifications of cycles, relying on an organisation of time across days, notably, the notion of a regular 28-day cycle and ‘peak fertile times’. Fertility biosensors are only relevant for a certain portion of one's life: the so-called ‘reproductive years’ (which can, of course, constitute many decades). Menstrual apps marketed to young women and those oriented towards tracking the (peri-)menopause help to constitute the beginnings and ends of this period of time, as well as shaping its contemporary unfolding as a biomedical phenomenon. In this sense, they directly participate in the figuring (that is, both the discursive formation and the lived experience) of women's (and potentially men's and others’) life courses more broadly. As the so-called new reproductive technologies did before them (Franklin, 2013), fertility biosensors, in other words, remake what it means to be sexed/gendered, to reproduce and, thus, to be human.

Reasons for monitoring fertility and associated flows of sex hormones are diverse, ranging from curiosity, through strong desires to avoid or achieve pregnancy, to medical concerns about gynaecological or reproductive health (endometriosis, polycystic ovary syndrome and other conditions), and the tracking of processes of gender transition. In this chapter, we explore practices of fertility and sex hormone biosensing, asking how they are changing enactments of contemporary bodies, sex/gender, reproductive relations and associated notions of ‘the reproductive years’.